1. Field of the Invention
The present invention relates to equipment protection and energy saving devices, and in particular, to a circuit arrangement for protecting equipment subject to damage by having a relatively low operating voltage connected thereto.
2. Discussion of the Relevant Art
Many circuit arrangements are known in the art which are designed to prevent damage to compressor motors and other appliances that could be damaged by the application of lower than normal operating voltages. Continuous application of voltage in excess of the normal rating of the equipment is considered rare and have not been of much concern to the inventor. Typical appliances which could readily be deteriorated by the application of less than the minimum rated operating voltage include refrigerators, air-conditioners and other motor-driven devices. The application of lower than specified voltages requires additional current from the source, thereby causing overheating of the motor and eventual burn-out.
In equipment that utilizes compressors associated with driving motors, turn-on of the equipment immediately after an instaneous shut-down causes the equipment to turn-on into an excessively heavy load and may also cause a threatening condition to the motor shortening its life. Typical consumer products are designed to operate from 115 volts RMS+10% at 60 Hz. This means that a minimal voltage of 103.5 volts is acceptable and the equipment should function properly over its normal life expectancy. However, should the line voltage fall to as low as 100 volts RMS overheating would occur in the equipment and possible damage could result. With excessive use of equipment, particularly during the warm summer months, the line voltage, because of overload conditions, has been known to drop much below the 103.5 volts RMS required for the operation of the equipment. Therefore, may circuit arrangements have been devised to remove the equipments from the line voltage during periods of extended time at the reduced or "brown-out voltage". Intermittent or transient low voltage swings have not been much of a problem since they rarely exist for any length of time.
Typical circuit arrangements may be found in U.S. Pat. No. 4,268,884 issued to Ford. Jr., et al on May 19, 1981, which disclose a current sensing circuit that controls a contactor that connects a load to power lines. The circuit arrangement provides an initial momentary delay period after energization of the circuit and before operation of the contactor to permit transients and other short duration irregularities to die out. An immediately succeeding second momentary delay period during which the contactor operation cannot be changed, assures that an initial inrush of current to the load will not cause a fault signal to erroneously operate the contactor. When a fault signal does cause the contactor to be operated, the circuit cannot be set back to its intended operation condition until energization of the sensing circuit is manually interrupted and reestablished. During normal operation of the circuit when current on the power lines is within acceptable limits, the circuit is latched in its proper operating mode.
U.S. Pat. No. 3,950,675 issued to Weber, et al on Apr. 13, 1976, relates to a motor protection device and discloses a circuit arrangement utilized to protect motors, compressors and the like, from conditions of low energizing voltage or momentary voltage interruptions resulting in excessive mechanical load (locked rotor). When the undesirable condition is present, an internal relay prevents the equipment from being energized, with transient conditions being allowed for by means of suitable time delay circuitry.
U.S. Pat. No. 3,619,668 issued to Pinckaers on Nov. 9, 1971, discloses a circuit arrangement which may be used for the control of equipment such as refrigeration compressors and provides for a minimum off-time control. The minimum off-time control includes a resistor-capacitor timing circuit which charges initially from a power source through a transistor, and this is held in a charged condition while the current continues to flow through the transistor to energize a solid-state switch which energizes the contactor supplying power to the equipment being protected. In the event that a momentary interruption of power occurs, or in the event that the power is removed, the capacitor starts to slowly discharge activating a second transistor circuit that shorts out the first transistor so that the output switch cannot be reenergized for a predetermined minimum period of time.
Prior art semiconductor devices utilized in the disclosed circuit arrangements have not had the reliablity desired and freqently, they would not operate at the predetermined threshold because the circuit arrangement was unreliable and/or damaged because of overvoltage transients appearing on the line prior to the undervoltage condition. The present circuit arrangement overcomes the shortcomings found in the prior art by providing overvoltage transient protection, as well as undervoltage or brown-out protection for equipment being protected by the present apparatus.